In today’s Academic Minute, Dr. Elizabeth Pringle of the University of Michigan reveals how some tropical trees pay armies of ants to defend them against herbivorous pests.
Elizabeth Pringle is an assistant professor in the Department of Ecology and Evolutionary Biology and at the School of Natural Resources and Environment at the University of Michigan. Her research seeks to understand how and why the costs and benefits of mutually beneficial interactions between organisms vary with environmental and historical context, and how such interactions shape ecosystems and coevolutionary processes.
Dr. Elizabeth Pringle – Trees and Ant Defenders
Ecuador laurel trees have friends indeed in the armies of tiny ants that defend them from leaf-eating insects. When water is scarce, laurel trees need the ants to defend them better against the pesky leaf-eaters, and that's just what the ants do.
Trees require water to make sugars, and both trees and ants require sugars for food. Ants can't feed on laurel-tree sugar directly, so they employ scale-insect "middlemen," vampires of the insect world who suck the tree's sugary sap and defecate a sugary meal for the ants. With a ready source of tree sugar to feed them, the ants return the favor by defending the tree's sugar-producing leaves. Ants patrol the surface of the tree and bite leaf-eating intruders until they go away.
Ecuador laurel trees are found in tropical forests, and forests are drier in Mexico than they are further south in Central America. Less water means a shorter growing season and less time for the trees to produce the vital sugars they need to survive the stressful dry season. For these water-stressed trees, leaves are life. So they "buy" ant insurance: trees at drier sites provide more sugar rewards for ants by supporting higher numbers of sap-sucking scale insects. The ant colonies respond by producing more aggressive ants who reduce the tree's risk of losing its leaves. At wetter sites, in contrast, trees that lose their leaves have time to produce new ones, so they don't need to invest sugar in ant insurance. The ants, in turn, are more reclusive at wetter sites, and use more of the tree sugar they do obtain on their own reproduction.
So, for laurel trees and ants, water scarcity promotes cooperation. Moreover, future climate change will change water availability, and thus also how friendly the interactions among trees, ants, and their scaly middlemen will be.